In the manufacture of semiconductors, one or more coating or etching processes may be used. One of such processes is a physical vapor deposition ("PVD") sputter coating process. In a PVD process, a heavy gas, for example, argon, is ionized in a vacuum chamber. The argon ions impact a target and sputter off target material atoms, for example, aluminum, that are, in turn, deposited on a substrate or wafer. Thereafter, an etching process is used to leave a predetermined pattern of the material on the wafer. In other PVD applications, the target is titanium, and one or more reactive gases, for example, nitrogen and oxygen, are bled into the chamber to form either titanium nitride or titanium oxynitride.
During the PVD process, the target material being sputtered is not only deposited on the wafer, but is also deposited on other surfaces and shields within the vacuum chamber. Over a number of sputtering cycles, the thickness of the sputtered material on the shields and other surfaces within the vacuum chamber continuously increases. Further, with each sputtering cycle, the chamber and the components therein experience a heating and cooling temperature cycle. After a period of time, as the coating of sputtered material thickens on the shields, it has a tendency with successive temperature cycles to flake off of the shields in the form of small particles that range in size from approximately 0.1 microns to approximately several microns. Once in the environment of the vacuum chamber, it is highly probably that a particle will be deposited on a the substrate being processed. Normally, the sputtering material is a conductor; and therefore, if the particle is deposited across terminals of a device or across conductive paths on the substrate, the resultant short circuit is substantially thicker than the coating. Therefore, the subsequent etching process to remove the coating will not remove all of the particle; and the remaining unetched particle may create a short circuit which make an associated device on the wafer unuseable. Therefore, those larger particles which flake off of shields and surfaces in the processing chamber have an adverse impact on and reduce the yield of the completed wafer devices.
Process yields can be improved if the shields and other surfaces in the processing chamber are cleaned prior to the time when excessive particles begin to flake therefrom. The cleaning of chamber surfaces and the removal and replacement of the shields is a time-consuming and expensive process during which the sputtering chamber is out of production. Therefore, the cleaning process is preferably conducted only when necessary. However, postponing the cleaning process to the point where particles begin flaking from the coated surfaces, thereby reducing yields, is more costly than the cleaning process. Ideally, the processing chamber should remain in production right up to the time immediately prior to particle flaking.
In the past, the processing time of the chamber 22 was measured in kilowatt hours; and based on experience, the chamber was scheduled to be cleaned after the passage of a predetermined number of processing kilowatt hours. However, predicting the optimum time to clean a processing chamber by tracking the kilowatt hours of processing does not provide the optimum production processing time between cleaning procedures.
To improve the prediction of when maintenance and cleaning should be performed, a particle monitor can be used. One such monitor is an external laser based monitor capable of detecting very small particles. However, the cost of such a monitor makes its application to all processing chambers impractical. Another, less expensive, in-situ monitor can also be used, for example, a particle sensor model No. 20SD, commercially available from High Yield Technology of San Jose, Calif. The in-situ particle monitor includes a particle sensor control connected to an electronic laser probe that is located in the vacuum chamber. With such a particle sensor, a laser beam illuminates the plasma of gas ions and provides a count of particles within the plasma that are within a range of particle sizes, for example, from approximately 0.3 microns to approximately 5.0 microns. The microns are counted over a predetermined sample time period, or sample window. The total number of particles counted during a sample window is compared to a set point representing a threshold particle count value; and if the count exceeds the threshold value, an alarm is provided by the particle sensor control. The set point value is empirically determined to provide the most effective time at which to initiate a maintenance and cleaning of the processing chamber.
There are two disadvantages in the current application of the in-situ monitor. First, the particle monitor is enabled by a control signal derived from the application of power to the target cathode of the processing chamber. When power is applied to the target, the plasma of ionized gas provides significant interference and noise to the in-situ particle monitor that masks particles that otherwise could be counted. The net result is that the monitor is not counting all of the particles that it is capable of accurately discriminating. Therefore, there are more particles in the processing chamber than indicated by the particle monitor, and those undetected particles have a substantially adverse impact on yields. Second, there are certain times when power is applied to the target, but a production wafer is not located in the processing chamber. Therefore, the in-situ particle monitor is operating and collecting data during time periods when production is not occurring. Not only does such operation waste valuable memory space in the particle sensor control but any particle alarms that occur during nonproduction periods are a distraction to personnel.
Consequently, there is a need for an improved system for controlling the operation of a particle sensor during the coating process.